1. Field of the Invention
This invention relates in general to sensors for magnetic storage devices, and more particularly to a method for providing a self-pinned differential GMR sensor and self-pinned differential GMR sensor.
2. Description of Related Art
Magnetic recording is a key segment of the information-processing industry. While the basic principles are one hundred years old for early tape devices, and over forty years old for magnetic hard disk drives, an influx of technical innovations continues to extend the storage capacity and performance of magnetic recording products. For hard disk drives, the areal density or density of written data bits on the magnetic medium has increased by a factor of more than two million since the first disk drive was used for data storage. Areal density continues to grow due to improvements in magnet recording heads, media, drive electronics, and mechanics.
Magnetic recording heads have been considered the most significant factor in areal-density growth. The ability of the magnetic recording heads to both write and subsequently read magnetically recorded data from the medium at data densities well into the gigabits per square inch (Gbits/in2) range gives hard disk drives the power to remain the dominant storage device for many years to come.
Important components of computing platforms are mass storage devices including magnetic disk and magnetic tape drives, where magnetic tape drives are popular, for example, in data backup applications. Write and read heads are employed for writing magnetic data to and reading magnetic data from the recording medium. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
A magnetoresistive (MR) sensor changes resistance in the presence of a magnetic field. Recorded data can be read from a recorded magnetic medium, such as a magnetic disk, because the magnetic field from the recorded magnetic medium causes a change in the direction of magnetization in the read element, which causes a corresponding change in the sensor resistance.
A magnetoresistive (MR) sensor detects magnetic field signals through the resistance changes of a sensing element as a function of the strength and direction of magnetic flux being sensed by the sensing element. Conventional MR sensors, such as those used as MR read heads for reading data in magnetic recording disk and tape drives, operate on the basis of the anisotropic magnetoresistive (AMR) effect of the bulk magnetic material, which is typically permalloy. A component of the read element resistance varies as the square of the cosine of the angle between the magnetization direction in the read element and the direction of sense current through the read element. Recorded data can be read from a magnetic medium, such as the magnetic disk in a magnetic disk drive, because the external magnetic field from the recorded magnetic medium (the signal field) causes a change in the direction of magnetization in the read element, which in turn causes a change in resistance of the read element. This change in resistance may be used to detect magnetic transitions recorded on the recording media.
In the past several years, prospects of increased storage capacity have been made possible by the discovery and development of sensors based on the giant magnetoresistance (GMR) effect, also known as the spin-valve effect. In a spin valve sensor, the GMR effect varies as the cosine of the angle between the magnetization of the pinned layer and the magnetization of the free layer. Recorded data can be read from a magnetic medium because the external magnetic field from the recorded magnetic medium, or signal field, causes a change in the direction of magnetization of the free layer, which in turn causes a change in the resistance of the spin valve sensor and a corresponding change in the sensed current or voltage.
Magnetic sensors utilizing the GMR effect are found in mass storage devices such as, for example, magnetic disk and tape drives and are frequently referred to as spin-valve sensors. The spin-valve sensors are divided into two main categories, the Anti-FerroMagnetically (AFM) pinned spin valve and the self-pinned spin valve. An AFM pinned spin valve comprises a sandwiched structure consisting of two ferromagnetic layers separated by a thin non-ferromagnetic layer. One of the ferromagnetic layers is called the pinned layer because it is magnetically pinned or oriented in a fixed and unchanging direction by an adjacent AFM layer, commonly referred to as the pinning layer, which pins the magnetic orientation of the pinned layer through anti-ferromagnetic exchange coupling by the application of a sense current field. The other ferromagnetic layer is called the free or sensing layer because the magnetization is allowed to rotate in response to the presence of external magnetic fields.
In the self-pinned spin valve, the magnetic moment of the pinned layer is pinned in the fabrication process, i.e., the magnetic moment is set by the specific thickness and composition of the film. The self-pinned layer may be formed of a single layer of a single material or may be a composite layer structure of multiple materials. It is noteworthy that a self-pinned spin valve requires no additional external layers applied adjacent thereto to maintain a desired magnetic orientation and, therefore, is considered to be an improvement over the anti-ferromagnetically pinned spin valve.
As systems are pushed to higher read density, higher magnetic bit size or decreased recording media size, the available magnetic flux is decreased. In addition, sensitivity may be decreased from thermal noise. For example, while the head is flying over the disk surface, it may hit a particle (contamination). The energy of this collision will be dissipated in the form of heat causing the temperature of the head to increase, causing an increase in the resistance of the head ultimately resulting in a signal that may be even higher than the magnetic signal from a transition. In order to sense these smaller signals and increase areal density, read heads with greater sensitivities are needed.
A scheme to increase the signal to noise ratio of a spin valve head is to employ first and second spin valve sensors, which are differentially detected for common mode noise rejection. A differential spin valve structure employs first and second spin valve sensors that produce responses of opposite polarities in reaction to a magnetic field of a single polarity. The opposite polarity responses are processed by a differential amplifier for common mode rejection of noise and for producing an enhanced combined signal. The first and second spin valve sensors are magnetically separated by a gap layer. The first spin valve sensor is connected in series with first and second leads and the second spin valve sensor is connected in series with third and fourth leads. The second and fourth leads are electrically interconnected and the first and third leads are adapted for connection to the differential amplifier.
While a differential GMR head provides an increased signal to noise ratio, the differential GMR head is significantly thicker than a single pinned spin valve sensor because of the thicknesses of the first and second pinning layers. While the thicknesses of the various layers of a typical spin valve sensor range between 10 Å-70 Å, the thicknesses of the antiferromagnetic pinning layers vary in a range from 120 Å-425 Å. Iridium manganese (IrMn) permits the thinnest antiferromagnetic pinning layer of about 120 Å whereas an antiferromagnetic pinning layer composed of nickel oxide (NiO) is typically 425 Å.
Further, the range of blocking temperature for the interface at the antiferromagnetic layer is relatively low. These temperatures can be reached by certain thermal effects during operation of the disk drive, such as an increase in the ambient temperature inside the drive, heating of the SV sensor due to the bias current, and rapid heating of the SV sensor due to the head carrier contacting asperities on the disk. In addition, during assembly of the disk drive the SV sensor can be heated by current resulting from an electrostatic discharge. If any of these thermal effects cause the SV sensor to exceed the antiferromagnet's blocking temperature the magnetization of the pinned layer will no longer be pinned in the desired direction. This will lead to a change in the SV sensor's response to an externally applied magnetic field, and thus to errors in data read back from the disk.
It can be seen that there is a need for a method for providing a differential GMR sensor and GMR sensor that is smaller and more sensitive.